P0135 Code on a Buick: O2 Sensor Heater Performance, Bank 1 Sensor 1 (Buick Guide)

P0135 on a Buick is GM's "HO2S Heater Performance, Bank 1 Sensor 1" — the electric heater inside the front oxygen sensor is drawing a current the engine computer does not expect. It is a heater fault, not a sign the sensor reads oxygen badly. On a Buick it matters more than the rear-sensor heater codes, because this is the sensor that runs the fuel mixture, and a cold front sensor keeps the engine on its rich warm-up fuel table for longer. The detail most pages miss: GM's own tables show the "normal" heater current range is different for each Buick engine, so a generic amp figure from the internet can send you the wrong way.

What Does P0135 Mean on a Buick?

An oxygen sensor only works hot. Until it reaches operating temperature, the engine control module (ECM) cannot use it, so the engine runs "open loop" on a pre-programmed, deliberately rich fuel table. Every Buick front sensor therefore carries an electric heater that gets it working within moments of a cold start.

The heater is powered through a fuse with ignition voltage, and the ECM switches the ground side with a low-side driver, pulsing it to hold temperature. P0135 is GM's performance check on that heater: the circuit may be intact, but the current flowing through it is outside the range the ECM expects for a healthy element.

GM's wording is "HO2S Heater Performance Bank 1 Sensor 1," and its OBD parameter tables label it "O2S Heater Performance." On older generic lists you may see "O2 Sensor Heater Circuit," which is less precise.

How GM Tests the Heater

GM's OBD parameter tables spell out the test, and they are the best evidence of why one-size-fits-all advice fails. In each case the ECM measures heater current through the low-side driver and sets P0135 when it falls outside a calibrated window:

• 2015 Encore 1.4L turbo (and the Regal, Verano and LaCrosse four-cylinder group): expected current between 0.3 and 2.5 amps.
• 2015 Enclave 3.6L and LaCrosse 3.6L: expected current between 0.3 and 1.2 amps.
• 2021 Encore and Encore GX group: expected current between 0.1 and 4.3 amps.

The rest of the test is consistent across Buick engines. It waits for the heater warm-up delay to complete, needs system voltage above 10 volts (11 volts in the 2021 table), the heater commanded on, and those conditions held for more than 120 seconds. It then runs two tests per trip and needs 8 failures out of 10 samples. It is a Type B code, so the light normally comes on after two consecutive failing trips.

Two practical consequences follow. First, a current reading that looks "fine" on one Buick might be a fault on another — use the figure for your engine. Second, because the test needs two minutes of steady running, a quick start-and-shut-off after a repair proves nothing.

Which Buicks Set P0135

GM's parameter tables list P0135 for every modern Buick engine group we checked:

• Encore 1.4L turbo (LUJ/LUV, later LE2) and Encore GX 1.2L/1.3L turbo
• Verano 2.4L and 2.0L turbo; Regal 2.0L turbo and 2.4L
• LaCrosse 2.4L, 2.5L and 3.6L
• Envision and Cascada
• Enclave 3.6L (LLT, and LFY from 2018)

On the four-cylinders there is one bank, so P0135 is the only front heater code. On the Enclave and LaCrosse V6, P0135 is the bank 1 front sensor; bank 2's front heater performance code is P0155.

What GM's Buick Bulletins Say

We did not find a GM bulletin that names P0135 on a Buick. Three GM documents are still directly relevant, because they cover the same sensor and its wiring:

2013–2015 Enclave 3.6L — PIT5336. GM lists the front heater circuit codes P0030, P0031 and P0032 (and the bank 2 equivalents) among about thirty oxygen sensor and purge codes, and says the Emission 1 fuse may be open. The cause is the bank 1 oxygen sensor harness chafing on the A/C hose coupling at the back of the engine. Since that fuse feeds the heaters, a front heater code on one of these Enclaves should send you straight to that harness.

2016–2017 Enclave 3.6L — PIP5516A. GM lists P0032 — heater control circuit high, bank 1 sensor 1 — with P0131, P0132, P0133, P0137, P0171 and P2096 as a "repeat oxygen sensor failure." The cause is power steering fluid leaking onto the bank 1 oxygen sensor harness. GM replaces the sensor, the power steering inlet hose and the engine harness.

2013 Encore 1.4L — recall 13292. Certain 2013 Encores were built with an incorrect front heated oxygen sensor. Dealers replace it. The recall does not name P0135, and its age means most involved cars will have been done, but a free VIN check is worthwhile on a 2013 Encore with any front-sensor code.

Where Bank 1 Sensor 1 Is on a Buick

Sensor 1 is upstream of the catalytic converter. On the turbocharged Encore, Encore GX, Regal, Verano and Envision it sits in the exhaust just after the turbocharger. On non-turbo four-cylinders it is in or just after the exhaust manifold.

On the 3.6L V6 in the Enclave and LaCrosse, bank 1 is the bank with cylinder 1 — in these transverse installations the rear bank, toward the firewall. Both Enclave bulletins place the bank 1 sensor harness at the back of the engine, which is also where it is hardest to see.

P0135 Versus P0030, P0031, P0032 and P0053

GM separates front heater faults into several codes, and knowing which one you have narrows the search:

GM's tables put P0030, P0053 and P0135 in the same bank 1 sensor 1 fault bundle as the signal codes P0131–P0134, and other tests such as P0131 on WRAF-equipped engines will not run while P0030, P0031, P0032 or P0135 is active. Fix the heater first.

Symptoms of P0135

• Check engine light, often after a cold start and a couple of minutes of driving
• Slightly worse fuel economy, especially on short trips
• Longer than normal warm-up before the engine settles
• Readiness monitors incomplete or a failed emissions test
• Other oxygen sensor or purge codes at once if a shared fuse has blown (2013–2015 Enclave)

Causes and How to Confirm Each

How Is P0135 Diagnosed on a Buick?

1. Record all codes. P0135 with P0155 (both front heaters on a V6) or many O2 codes together points at a shared fuse or supply.
2. Check the VIN for PIT5336, PIP5516A or recall 13292.
3. Check the fuse feeding the heaters. If it is open, find the short before replacing it.
4. Measure supply voltage at the sensor connector with the key on.
5. Inspect the harness and connector — on an Enclave, behind the engine at the A/C coupling and under the power steering hose.
6. Check heater resistance at the sensor and compare with the service information figure for your engine.
7. Watch heater current or status on a scan tool and compare it with your engine's range, not a generic number.
8. Replace the sensor only when supply, ground and wiring test normal — with the correct part for your VIN.
9. Verify with real drives: the test needs over two minutes of steady running and runs twice per trip.

Parts and Cost

GM Documents Relevant to P0135 on a Buick

None of these GM documents names P0135 on a Buick. Have a dealer check your VIN against current bulletins rather than relying on any article, including this one.

What Other P0135 Pages Get Wrong

• Quoting one heater current figure for every car. GM's own Buick windows range from 0.3–1.2 A on the 3.6L to 0.1–4.3 A in the 2021 Encore group.
• Leading with the sensor. Fuse, supply voltage and connections come first; a weak supply can set a performance code on a good sensor.
• Testing with one quick restart. GM's test needs more than two minutes of qualifying conditions.
• Ignoring the Enclave harness bulletins. Two GM documents put front-sensor wiring problems on the Enclave's bank 1.
• Treating every front sensor as interchangeable. Buick engines use different front sensor designs; order by VIN.

Frequently Asked Questions

Can I drive my Buick with P0135?

Yes in the short term. The sensor still warms from exhaust heat eventually, but the engine stays in open loop longer on every cold start — more fuel used and higher emissions. Fix it before inspection time.

Is P0135 the front or rear sensor?

Front — Bank 1 Sensor 1, before the catalytic converter. The rear sensor heater on the same bank sets P0036 or P0141.

Which bank is bank 1 on an Enclave or LaCrosse V6?

The bank with cylinder 1, which in these transverse installations is the rear bank, toward the firewall.

Why does the light come on a few minutes after startup?

GM's test waits for the heater warm-up delay and more than 120 seconds of qualifying conditions, then tests twice per trip. As a Type B code it usually needs two failing trips.

Does an Encore have a bulletin for P0135?

We found none naming P0135. Certain 2013 Encores were recalled (13292) for an incorrect front oxygen sensor, which a dealer can check by VIN.

What heater current is normal on my Buick?

It depends on the engine. GM's tables give 0.3–2.5 A for the 2015 Encore 1.4L, 0.3–1.2 A for the 2015 3.6L V6 and 0.1–4.3 A for the 2021 Encore group. Use your engine's service information.

Where can I read the general version?

Our GM P0135 guide covers the code across GM brands.